Carboniferous mineralisation, Wales

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From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

The lead-zinc (Pb-Zn) mineralisation in Central Wales Mining District, between Aberystwyth and Llanidloes, is hosted mainly in Silurian and, less commonly, Ordovician rocks. However, lead isotope data indicates that the mineralising fluids were initially active in early Devonian times, possibly in response to the Acadian phase of the Caledonian Orogeny, and later in Carboniferous times, possibly prior to coalification events in south Wales. Similar associations occur in Silurian strata, in the Llanfair Talhaiarn district in Denbighshire, and in Ordovician strata in the Llanrwst district, on the west side of the Conwy valley.

In north-east Wales, there is extensive mineralisation along Variscan faults within Lower Carboniferous strata; on Halkyn Mountain and in the vicinity of Minera, the mineralisation has been intensively exploited. The deposits are mainly concentrated in the Lower Carboniferous limestones and the overlying Cefn y Fedw sandstone with lesser concentrations in the Basement Beds. The ores are mainly of sphalerite and galena, with little silver, associated with chalcopyrite, baryte and fluorspar. The ores are concentrated along joints, faults and, in the limestones, in cavities along bedding planes where the calcite has been dissolved. The ores tend to be richest where capped with a bed of shale, which restricted the upward movement of the solutions. The solutions are assumed to be magmatic, although their source remains problematic as there is no indication of contemporaneous igneous activity.

In south Wales, iron mineralisation in the Lower Carboniferous limestones has been extensively worked in the Vale of Glamorgan, near Llanharry. The ore consists mainly of colloidal and crystalline hematite, commonly altered to goethite, with quartz, calcite and dolomite. The ore bodies occur along the north-north-west-trending cross-faults. They fill large cavities and enlarged fractures in the limestones, generally beneath the Triassic unconformity, close to their contact with impermeable Namurian mudstones. The ores have been related to hydrothermal and to meteoric groundwater solutions. Probably both solution processes were involved — an earlier hydrothermal (sulphide) phase and a later replacement phase due to meteoric circulation. With the latter, the iron and silica are thought to have been leached from Triassic and Silesian strata, and deposited both as replacements and in cavities.

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